The invention relates for example to an LED module which is designed to provide uniform illumination of a planar surface. However, the invention can be applied generally to beam shaping optics.
One known approach to achieve a planar surface illumination is to use a so-called batwing intensity distribution (also referred to as a wide beam intensity distribution). The term batwing refers to a highly peaked shape of the intensity distribution in a polar plot.
FIG. 1 shows an example of a batwing intensity distribution as a polar plot. The two wings 10, 12 in this example have a peak intensity at 60 degrees each side of the normal, and the aim is to provide a uniform surface illumination over the full 120 degree range. The intensity is higher at the grazing angles because the surface area being illuminated per unit angle increase steeply.
The ring 14 is the light intensity in a perpendicular direction. For a rotationally symmetric light distribution this would be a batwing distribution as well. For a linear light source it is for example a circle (i.e. Lambertian), distribution.
To create the desired batwing profile from an LED, an optical component is required to compensate the well-known cosine fourth law which applies to a Lambertian point source (by which illuminance falls following a cos4 ϑ function). The optical design thus needs to change the Lambertian intensity distribution from an LED output intensity into the batwing distribution.
The batwing light distribution allows for a uniform illumination of a planar surface for example even up to a 140° beam angle. Such light distributions and hence lens designs are used for example in street lighting and wall washer applications. In these examples, the batwing distribution targets a planar surface in the far field: the illuminated surface is positioned at a distance much larger than the light module dimensions.
Similar batwing distributions may also be applied to illuminate the interior of a luminaire housing, e.g. the exit window of a luminaire. This would then create a spatially uniform luminescent panel. This gives a desired uniform appearance of the luminaire itself rather than an illuminated surface. A known alternative approach for increasing spatial uniformity in a luminescent panel is by extensive scattering: using reflective matte white surfaces at the inner side of the casing or well-designed white paint dot patterns on a light guide.
Scattering based solutions typically allow for a high spatial uniformity at the expense of efficiency and/or form factor. Moreover, the light distribution at the exit window will be limited to a Lambertian distribution at each position of the surface, while an optical element with a batwing design may instead assign a constant flux to each position from a known direction, i.e. the light source position. This allows for further beam shaping at the exit window position.
There are two known designs of lens capable of changing a Lambertian intensity distribution into a batwing intensity distribution.
A first example is a so-called peanut design as shown in FIG. 2 and a second example is a so-called bubble optic as shown in FIG. 3.
The difference in shape is determined by the choice of ray deflecting surface. The surface changing the Lambertian distribution into a batwing is for the peanut optic the outer lens surface while for the bubble optic it is the inner surface.
Mass manufacturing technology for such lenses is for example based on printing or injection moulding, where the cost price of a single optic is primarily determined by the amount of plastic and its cycle time. Also, the cycle time scales with the amount of plastic. Hence, in order to provide cost effective solutions, a design is desired that uses the least amount of material.
There is thus a need for a lens design for an LED design which enables a reduction in the amount of material needed.